Well known in the art are solid golf balls, for example, two-piece golf balls which are generally formed by milling polybutadiene rubber, methacrylic acid or another unsaturated carboxylic acid or a salt thereof, zinc oxide, peroxide and any desired additives together to form a rubber composition, thermoforming the composition into a solid core, and covering the core with a cover of a thermoplastic resin. In these solid golf balls, for example, two-piece golf balls, hard ionomer resins having excellent cut resistance are generally used as the thermoplastic resin to form the cover. The solid golf balls having a cover formed of such an ionomer resin exhibit improved durability and flying performance. In order to impart further improved repulsion or initial speed upon impact to the solid golf balls, it is proposed to control the cover of the ionomer resin to a thickness of about 22.1 to about 2.5 mm. See Japanese Patent Application Laid-Open Nos. 59-37961 and 59-49780.
As compared with conventional thread-wound balls, these solid golf balls, however, show markedly inferior controllability and unpleasant feel when they are hit with a middle or short iron requiring accurate control as well as flying performance. For this reason, few professional golfers use solid golf balls in current games.
Some attempts were made in the past to render the hitting feel of solid golf balls soft by reducing the hardness of the solid core or by molding the cover from a softer material. These attempts failed to produce satisfactory balls because flying performance and durability were more or less deteriorated.
On the contrary, wound golf balls are currently used by most professional golfers in games because the wound golf balls are superior in flying performance and controllability. However, the wound golf balls have poor durability including cut resistance and fatigue endurance. Further, most ordinary players fail to make the best use of the wound golf balls to fully enjoy the superior flying performance thereof, resulting in a rather short flying distance. Thus most ordinary players like solid golf balls such as two-piece golf balls which exert the excellent flying performance even when hit with ordinary skill. For the purpose of improving the flying performance of wound golf balls, it was proposed to use a lightweight center having a relatively large diameter and coat the center with a high specific gravity cover to produce a ball having increased inertia moment (see Japanese Patent Application Laid-Open No. 59-129072). However, such attempts have been unsuccessful in improving the flying performance and durability to an extent comparable to solid golf balls as exemplified by two-piece golf balls.
Therefore, there is the need for golf balls which can fly a long distance comparable to solid golf balls and exhibit excellent controllability comparable to wound golf balls, as well as pleasant hitting feel and high durability.